1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more networks. A base station (or alternatively a node-B, base station router, or access network) typically provides wireless connectivity to mobile units located in each cell. The mobile units may include devices such as mobile telephones, personal data assistants, smart phones, text messaging devices, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. Mobile units located in each cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with the base station that serves the cell.
The performance of a wireless communication system is typically determined by a signal-to-noise ratio (SNR) associated with signals received by the mobile units. The noise received by the mobile units includes thermal noise and interference from transmissions associated with other mobile units and/or base stations. The interference from transmissions associated with other mobile units within the same cell is typically fairly small because the downlink radio channels are substantially orthogonal to each other. For example, downlink transmissions to mobile units within the same cell may be encoded with orthogonal coding sequences. However, interference from transmissions associated with mobile units in adjacent cells may be significant, at least in part because the downlink radio channels used by base stations that serve adjacent cells are not typically orthogonal. Compared to mobile units located near the center of a cell, mobile units located near the edge of the cell may receive relatively weaker signals from the serving base station and relatively more inter-cell interference from base stations in neighboring cells. Thus, mobile units near the cell edge are likely to experience the largest degradation in the signal-to-noise ratio.
Reducing the maximum transmission power for base stations in adjacent cells may reduce the inter-cell interference experienced by mobile units near the cell edge. For example, the average power per channel could be reduced while maintaining the same number of radio channels, e.g., by scheduling high-speed data users on relatively high quality radio channels that require less energy per symbol than relatively poor quality channels. However, reducing the average power per channel is equivalent to shrinking the overall cell coverage associated with the base station. For another example, the number of radio channels could be reduced while maintaining the average power per channel. However, reducing the number of radio channels is equivalent to reducing the overall system capacity.